What is it?
The shutter is the door separating your image sensor from the light let in through the aperture of your lens. It snaps open and shut with the touch of your finger and makes that fabulous noise that lets you know your camera's doing it's job! While the aperture controls how much light hits your image sensor via the size of it's pupil-like opening, the shutter controls how much light hits your image sensor via how long it stays open.
How is it measured?
Shutter Speed (how long your shutter stays open) is measured in fractions of seconds. Yes, that's all it takes to make images that you will have for a lifetime - fractions of seconds. You'll see it represented like this: 1/25, 1/80, 1/125, 1/250, etc. (depending how sophisticated your camera is the increments will vary slightly but the concept remains the same)
Once again, no offense to the Einsteins among us, but this is another one of those upside down and backwards photography science gems. Our advice is to ignore the whole fraction thing and simply pay attention to the bottom number. The smaller the number, the slower the shutter speed (the longer it stays open and the more light is let in). The bigger the number, the faster the shutter speed (the faster it closes and the less light is let in). It works, we promise!
What does it mean to you?
The shutter, like aperture, accomplishes two different things.
First, it too has a deciding factor in how much light hits the sensor. It's the gatekeeper of the image sensor and how long it stays open determines how much of the light let in t hrough the aperture will hit the image sensor. A slow shutter speed lets in more light because it's open for longer - exposing the image sensor to light for a longer period of time. A fast shutter speed lets in less light because it's open for a tiny amount of time - exposing in the image sensor for a shorter period of time.
Second, like aperture, the shutter also has a creative impact on pictures - though an entirely different one that has to do with motion. Slower shutter speeds (small bottom numbers) allow you to show motion while faster shutter speeds (large bottom numbers) freeze it.
In general when photographing children or any other moving target, you are going to want to stick to this rule of thumb: Your shutter speed should be AT LEAST double the focal length of your lens to avoid motion blur. So if you are using a 50mm lens, then try and stay above 1/125. If you are shooting at 85mm, try and stay above 1/200 and so on. Don't ask why - just trust us! It goes a long way to eliminate that miniscule blur that makes your picture look annoyingly out of focus.
Now there is a time and place for slow shutter speeds. Wanna show your superstar sliding into first base? Or how about turning that tiny tot on the swing into a rainbow of blur? Anytime that you want to show motion to tell the story. Looking at the picture below, it's hard to forget exactly how this themepark ride felt - this action was caught at just 1/4 of a second!
Night photography is also a great time to experiment with slow shutter speeds, not because of motion, but in order to let in enough light to record the scene. Wanna turn the night sky into your own Goodnight Moon memory? Or remember what your Christmas tree looked like lighting up the room? Grab a tripod and use a reaaaallly low shutter speed number - maybe more than a second???
And what about super high shutter speeds? Pull out that camera on a rainy day and watch those raindrops freeze in time! Or better yet, wait for that mid-summer sprinkler romp and you'll appreciate why freezing motion with a high shutter speed can be such an important storytelling trick! We can see drop by drop how this sweet little boy got his picture-taking outfit wet at a shutter speed of 1/1000.
The shutter speed works both to determine the amount of light that will hit the image sensor and to let you decide whether you want to freeze or show motion.
Small bottom number = slow shutter speed = show motion
Large bottom number = fast shutter speed = freeze motion